This invention relates to a process and apparatus for forming flexible fold lines in single-layer thermoplastic sheets and to a box blank formed thereby. The invention is particularly useful in making folded box blanks. According to the invention, surface-limited heat is applied until the sheet cross-section becomes at least partially plasticized and thereafter the sheet is deformed between an edge forming tool and a counter tool. The prepared fold lines are made in an initially mainly flat sheet and make it possible, at a later time, to bend the sheet sections adjacent the fold line to form an edge or corner. Thus, the invention does not deal with the production of a permanent bend in the material.
For a better understanding of the differences, it should be noted that the invention relates primarily to the manufacture of box blanks. At a later time, these box blanks are folded into closed hollow bodies joined together at the circumference. The boxes, however, remain flat until they are filled with goods. The boxes are usually packed in a different location than where they are produced and are therefore delivered flat in order to save space. One example of a transparent package which meets the above requirements is described in the German Registered Design No. 1,937,659.
These folded flat boxes are preferably unfolded using automatic equipment. Considerable difficulties have been experienced, however, since the boxes tend to spring back into an intermediate position between their flat position and their upright or box-shaped positions thereby creating great difficulties in packing and closing the boxes. This spring back phenomena is caused by the high elastic deformity at the fold lines or bent edges during the unfolding of the box blanks.
In an attempt to overcome this problem, it has been proposed to bend the box blanks past their unfolded positions during the setup of the boxes so that when the boxes spring back, they will spring back into the proper position. A device capable of performing this operation is described in German patent No. 1,938,938. These devices, however, are not easily compatible with standard cardboard machines for packing and closing folded boxes. They therefore have limited use and accordingly are undesirable.
One method for producing fold lines in a thermoplastic sheet is described in German patent No. 2,236,617. According to this method, a knife edge is resistance heated to a temperature above the melting temperature of the plastic and the edge is pressed into the plastic sheet. Contrary to the explanation given in the patent, it has been found that a spring back resilience in the folded edges cannot be avoided to the degree desired. This is apparently due to the fact that the highest temperature is found along the contact surface between the knife edge and the plastic. From there, the temperature decreases towards the inside of the plastic material.
A method for bending thermoplastic sheets is also described in the book "Heat Sealing of Plastics" by W. A, Neitzert, Zechner and Heuthig Publ. GmbH, Speyer/Rhein, 1972. On pages 60 and 61 of the book, a method is described by which a heated knife is pressed into a thermoplastic sheet thereby melting the surface of the sheet. As with the previously described method, the highest temperature is found along the contact surface between the knife and sheet. According to this process, immediately after the heating knife is removed from the sheet, the sheet is bent so that the still molten surfaces contact each other and are sealed. Thus, permanent folds or bends are created by this method in contradistinction to the present invention which produces foldable or collapsible edges.
Another method for forming bends in thermoplastic sheets is described in "High-Frequency Welding" by G. F. Abele, Zechner and Heuthig Publ. GmbH, Speyer/Rhein, 1973. Pages 222-224 of this book describe a method by which a thermoplastic sheet is heated between high frequency electrodes along a fold line. An impression is then made along the fold line. However, the impression is explicitly limited to 20% of the material thickness. Furthermore, the plastic sheet is removed from the mold immediately after the high frequency heating and is immediately deformed by bending. Thus, this process produces a permanent and not easily removable bend in the thermoplastic sheet.